The invention relates to a method for the treatment of alloy steels and refractory metals such as Ti, Zr and Nb, for example, especially for depassivation and for subsequent thermochemical surface treatment in a process chamber under the action of pressure and temperature.
In the thermal treatment of surfaces (e.g., nitriding, carbonitriding or boriding) of alloy steels and refractory metals (e.g., Ti, Zr, Mo, W, Nb, Ta, V) the following difficulties have long been encountered due to the passive coatings covering the surface of the materials: The passive coatings consist usually of oxides and form a thin protective skin which disadvantageously prevents the unhampered diffusion of nonmetals, such as N, C and B, for example, during surface treatment. Consequently, in the case of the refractory metals, for example, diffusion is entirely prevented, and in the case of high-alloy steels it is partially prevented, resulting in irregular results.
In certain types of alloy steels a preoxidation is performed in order to achieve a uniform result of treatment. Impurities on the surfaces are thus oxidized and the pre-existing oxide coating is influenced. In many cases, therefore, the uniformity of the coating can be influenced. The coatings produced are very thin and always contain greater amounts of oxygen.
Now, it is the object of the present invention to condition the surfaces of the said materials by pretreating them such that an unhampered absorption of diffusible atoms will be possible during thermochemical treatment.
This object is achieved in accordance with the invention performing a treatment in several steps.